Imaging of ionospheric plasma outflow in the magnetosphere: Verification of a new concept

Imaging of ionospheric plasma outflow in the magnetosphere: Verification of a new concept

Adv. Space Res. Vol. 6, No. 3, pp. 215—220, 1986 0273—1177/86 $0.00 + .50 Copyright © COSPAR Printed in Great Britain. All rights reserved. IMAGING...

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Adv. Space Res. Vol. 6, No. 3, pp. 215—220, 1986

0273—1177/86 $0.00 + .50 Copyright © COSPAR

Printed in Great Britain. All rights reserved.

IMAGING OF IONOSPHERIC PLASMA OUTFLOW IN THE MAGNETOSPHERE: VERIFICATION OF A NEW CONCEPT S. Chakrabarti,* J. C. Green,’~Y. T. Chiu,** R. M. Robinson,** G. R. Swenson** andD. S. Evans*** *Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA. * Space Sciences Laboratory, Lockheed Palo Alto Research Laboratories, Palo Alto, CA 94304, U.S.A. * * * Space Environment Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303, U. S.A. ABSTRACT The distribution of He~,O~,and charge-exchanged auroral protons can be obtained by tomographic deconvolution of resonantly scattered solar 304, 834 and 1216 A signals, respectively, by these ions in the sunlit magnetosphere. The Doppler shifts of the emission features, if any, can be used to infer the energies of these ions. This concept have been verified for the case of plasmaspheric He~ions by the Extreme Ultraviolet telescope on board the Apollo-Soyuz mission. Our calculations for the 011834A line are in agreement with the observations by the STP78-1 satellite experiment. One Spacelab I experiment has observed “hot emissions” near 121 oA which are predicted by the charge exchange of auroral protons with geocoronal neutral hydrogen atoms. We discuss results of a simulation ofexpected intensities from various observing conditions and describe the design of an instrument capable of verifying the hypothesis. INTRODUCTION One of the recent advances in magnetospheric physics has been the discovery of ionospheric plasma in the magnetosphere. Although the global auroral imagery by the Dynamic Explorer (DE) /1/, HILAT /2/ and recently the Viking satellites have provided important information, they cannot image the ionospheric plasma in the magnetosphere. Advances in the Extreme and Far Ultraviolet (EUV and FUV) instrumentation (see for example Bowyer et al., 1981 /3/ and references therein) have paved the way for imaging global ionospheric oufflow through the observation of EUV and FUV solar lines after they are resonantly scattered by these ions or, in the case of hydrogen, charge-exchanged neutrals. In this paper we present observations which verify this concept In order to show that this imaging is feasible with present technology, we present the optical design of a rocket instrwnent which meets the requirements for such an experiment. THE CONCEPT Our concept of the proposed imagery originated with the interpretation of He304 data obtained by the EUV telescope on board the Apollo Soyuz Test Project (ASTP) /4/. The observed signals in a viewing geometry schematically shown in Figure 1 were shown to be in excellent agreement with a theoretical prediction based on resonant scattering of solar He304 line by He~ions along the line of sight The predicted He~density was obtained by using a kinetic model of the plasmasphere 151 with the ionospheric densities constrained by simultaneous measurements of He~density by the AE-C satellite. This concept of obtaining the densities of ionospheric ions in the magnetosphere by remote sensing techniques has recently been verified for the case of O~ions /6/. The relevant data were obtained by the U.C. Berkeley EUV spectrometer on board the STP78-1 satellite /3/ as shown for an orbit on March 22, 1979 in Figure 2. The 011834A line which was shown to be a sensitive indicator of auroral emissions 171, detected a bright auroral arc centered around —45000s UT between —50° and —55° geographic latitudes. We have obtained intensities observed for this spectral line just preceding and following the arc while the satellite was in the Earth’s shadow. The resulting zenith angle profiles are shown in Figure 2, as well as the detection of the emissions in both cases. From the anisotropy of the observed zenith angle dependence observed in the two regions, it is clear that the instrument has detected a significaàt emission source at altitudes greater than 1200 km. We believe that the observed 1-20 R signal obtained from altitudes as high as 2000 km is produced by resonant scattering of the solar and auroral O11834A line by upfiowing 0~ions along the auroral flax tubes. We have tested this hypothesis by modelling the distribution of such ions using a model of the auroral flux tube /8/. Using typical model parameters we obtain a predicted intensity for the O11834A emission of 6R 16/, which is quite consistent with the observation. With our concept of imaging the magnetospheric ion distribution, we cannot directly obtain the proton distribution in a manner similar to He~and O~ions. However, it should be possible to measure upflowing neutral hydrogen atoms created by charge exchange of H~with exospheric neutrals, using similar resonance scattering techniques. The }3Lyre emission at 1216A is the most obvious candidate for such measurements. However, since it is also the brightest spectral feature below 2000A special care must be taken. Because the upifowing H* ions will have energies ~ 1 eV, which is typical for exospheric neutral hydrogen, the resonant scattered signal due to these “hot” hydrogen atoms will be Doppler shifted from the cold geocoronal line core. In order to obtain the distribution of these charge-exchanged H atoms it will be necessary to measure the emission due to the hot hydrogen. The existence ofDoppler shifted H11216A magnetospheric emissions were tentatively verified from Spacelab 1/9/. The experiment used a H 2 gas absorber to measure the intensity of HLyU outside the central core. As shown in Figure 3, it measured 215

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—350 R emissions which cannot be attributed to either the interplanetary or any astronomical source. We have used the location of this emission along with near-instantaneous auroral locations indicated by two other satellites (NOAA-7 and NOAA-8) to verify our auroral hot 3hydrogen hypothesis. As shown in Figure 4, the abtolute intensity and extent can be accounted for by and a scale height of—1000km. using a density of 30cm

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Fig. 4. Comparison of hotHLya emission profile (solid curve) with the model (dashed curve). Note that the model is nonnalized to unity at the closest approach of the Spacelab I to the equatorward boundary of the auroral flux tube in the zenith direction. The model assumes that the hot hydrogen atoms are produced by charge exchange of upflowing H~ions and exospheric neutrals.

A SIMULATION In order to test the feasibility of global imaging of upfiowing ionospheric ions in the magnetosphere, we have run a model of as shown Figure 5.intensity The model /9/ electric field and plasma distribution for a discrete arc. In the simulations, we have 4RE investigated theinexpected for the assumes totalonly. potential drop ofare I keY andfor O~ of 500cm5 and(S) H~ density at of70cm3 at 200 km. for an average auroral O1I834Aaline The results shown andensity observation platform situated condition. The solar flux is assumed to be 1.3 kR. An additional auroral source of 1 kR /10/ at 100 km is also included in the modeL As can be seen from this Figure, the expected intensity varies from <1 R in the Outer magnetosphere to —l kR in the lower ionosphere. Note that the simulation is applicable to a situation for the minimum outflow of an inverted-V. Inclusions of conics and polar wind O~ions will increase the expected 011834A intensity. Similar to the l{Lya emissions these lines will be Doppler shifted, although the shift will be significantly smaller due to the higher mass of the O~ions. When considering an experiment tu measure the intensity of the magnetospheric 011834A emissions, it is important to note that the downlooking intensities from a platform at 4R 5 altitude will he cluttered by dayglow (—0.5 hR /11/) and nightglow (—1 R /12/) emissions.

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4RE. The O~densities at various locations along the Fig. 5. Simulated O11834A intensity profile for an observer located at flux tube are shown in the nightside lobe. The expected signal levels in various look directions are shown in Rayleighs.

A PROPOSED INSTRUMENT As a proof of concept experiment, we have designed a high resolution airglow spectrometer which fits within a standard Black Brant rocket payload. This instrument was designed with the intent of measuring the doppler shifted components of the Lyre at an energy resolution of 10 eV which represents a resolution of 0.2A at 1216 A. To detect the shifted Lya component, given a 10 Rayleigh intensity and a nominal rocket trajectory, we require a sensitivity of approximately 0.5 counts/sec/Rayleigh. We wish to measure the intensity ofthe O~834A emission simultaneously. The lower concentration and higher mass of the O~ion means that the intensity of the shifted component and its separation from the central line are greatly reduced. A measurement of the shifted component of 0~would therefore require significantly higher resolution and sensitivity. A Rowland circle mount employing a toroidal grating to improve the imaging represents the most efficient means of meeting these criteria. A Wadsworth system was also considered and was discarded because it reduces the sensitivity due to the limited throughput of the required mechanical collimator. In order to achieve the required resolution with line densities that can be fabricated with today’s technology, we found it necessary to work in at least the 3’~order. We have chosen to look at -3” order 12l6A and _ 4Ih

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In order to minimize the number of required components, we first considered a design using two slits, one grating and detector. However, this configuration does not allow sufficient space for the employment of the required detector inside a Black Brant envelope. A two inch diameter detector is required to accommodate both spectra. If the two slits are spread enough to incorporate the detector, then resolution and efficiency are unacceptably degraded. The next level of simplicity is to use one slit, one grating and two detectors. This system was found to perform exceliently. The resolution delivered is &~= .2A at 1216A , and &~= .3A at 834A . Total spectrometer efficiency is approximately 1 count/second/Rayleigh at each wavelength. In Figures 6a, 6b and 6c we show the results for monte carlo raytraces for the entire system. A raytrace at 11 wavelengths, from 1206 to l226A is displayed in Figure 6a. Note that this entire coverage is achieved in less than 1 inch of detector space, and that the images are virtually the same height as the slit. In Figure 6b we show a raytrace of 1216 and 1216.4A. The images are separated by twice their width, indicating that the actual resolution is 0.2A. A raytrace of the 834A detector is shown in Figure 6c. Here we have traced 830 to 840A with a &~of 1A. Here the separation is three line widths, so that the actual resolution is 0.3A. In all cases, the quoted dX’s cover at least two detector resolution elements, so that the resolution is not detector limited. Finally, in Figure 7, we show a scale schematic ofthe entire instrument, and how it fits within the typical Black Brant rocket envelope. CONCLUSIONS

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A reinvestigation of existing data has demonstrated that remote sensing of ionospheric plasma using resonant scattering sunlight has already been performed for H~,He~ and O~ions. We have also demonstrated that the observation of hot}{Lyce sions from Spacelab I can be explained by resonant scattering of solar HLya by charge exchanged auroral protons. Finally, we have described the optical design of a rocket-borne instrument to measure the density and energy distribution of these ions in the magnetosphere.

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5. Chakrabarti et al. ACKNOWLEDGEMENTS

This work was carried out with the NSF Grant ATM 8515008, and U. S. Army Research Office Contract DAAG29-85-K-0248.

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6. 7. 8. 9. 10. 11. 12.

REFERENCES L. A. Frank, J. D. Craven, I. L. Burch, and 1. D. Winningham, Polar views of the Earth’s aurora with Dynamics Explorer, Geophys. Res. Letr., 9, 1001, 1982. C. -I. Meng and R. E. Huffman, Ultraviolet imaging from space of the aurora under full sunlight, Geophys Res. Left., 11, 315, 1984. S. Bowyer, R. Kimble, F. Paresce, M. Lampton, and 0. Penegor, A continuous readout EUV airglow spectrometer, Appi. opt., 20,477, 1981. S. Chakrabarti, F. Paresce, S. Bowyer, Y. T. Chiu, and A. Aikin, Plasmaspheric helium ion distribution from satellite observations of h304, Geophys. Res. Left., 9, 151, 1982. Y. T. Chiu, I. G. Luhmann, B. K. Ching, and D. I. Boucher, Jr., An equilibrium model of plasmaspheric composition and density, J. Geophys. Res., 84, 909, 1979. Y. T. Chiu, R. M. Robinson, G. R. Swenson, S. Chakrabarti, and D. S. Evans, Concept and verification of imaging the outflow of ionospheric ions into the magnetosphere, Nature, in press, 1986. F. Paresce, S. Chaicrabarti, R. Kimble, and S. Bowyer, The 300- to 900A spectrum of a nightside aurora, J. Geophys. Res., 88, 10247, 1983. Y. T. Chiu and M. Schulz, Self-consistent particle and parallel electrostatic field distribution in the magnetosphericionospheric auroral region, J. Geophys. Res., 83, 629, 1978. 1. L. Bertaux, F. Goutail, and 0. Kockarts, Observations of Lyman re emissions of hydrogen and deuterium, Science, 225, 174, 1983. F. Paresce, S. Chakrabarti, S. Bowyer, and R. Kimble, The EUV spectrum of dayside and nightside aurorae: 800 — 1400A, J. Geophys. res., 88, 4905, 1983. S. Chakrabarti, F. Paresce, S. Bowyer, R. Kimble, and S. Kumar, The extreme ultraviolet day airgiow, J. Geophys. Res., 88, 4898, 1983. S. Chakrabarti, R. Kimble, and S. Bowyer, Spectroscopy the EUV (350 — 1400 A) nightglow, J. Geophys. Res., 89, 5660, 1984.

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